The present invention relates to a process for efficiently obtaining a mixed gas of a lower olefin and a lower aliphatic carboxylic acid by gasifying a lower aliphatic carboxylic acid at a low temperature. The mixed gas obtained by this process is useful particularly in the process for producing a lower aliphatic ester, where a corresponding lower aliphatic ester is obtained from a lower olefin and a lower aliphatic carboxylic acid in a gas phase in the presence of an acidic catalyst. By virtue of this, a production process of a lower aliphatic ester, having excellent effect on the energy cost, can be provided.
A method of obtaining a corresponding lower aliphatic ester by reacting a lower olefin and a lower aliphatic carboxylic acid in a gas phase in the presence of an acidic catalyst such as a heteropolyacid has been heretofore known. Specific examples include those described in Japanese Unexamined Patent Publications No. 4-139149, No. 5-170699, No. 5-294894 and No. 9-118647. These production processes are methods of reacting a lower olefin and a lower aliphatic carboxylic acid by passing them over an acidic catalyst in a gas phase.
Among these, the catalysts described in Japanese Unexamined Patent Publications No. 5-294894 and No. 9-118647 are so-called supported catalysts in which a heteropolyacid and/or a heteropolyacid salt as the catalytically effective component is supported on a porous substance so as to increase the catalytic activity. However, in a gas-phase reaction using a supported catalyst comprising an acid catalyst such as heteropolyacid supported on a solid catalyst, when the reaction mixture in a liquid droplet form, namely, in a mist state is contacted with the solid catalyst, the catalytic component may flow out from the support. As a result, there arises a risk that the catalyst is deactivated within a short time or the catalytic component is mixed in the product and causes an unpredictable side reaction.
In order to avoid such a risk and stably perform the reaction over a long period of time, a raw material compound which is liquid at an ordinary temperature must be completely gasified during the reaction using the catalyst. The same applies to the production process of a lower aliphatic ester, where a corresponding lower aliphatic ester is obtained from a lower olefin and a lower aliphatic carboxylic acid in a gas phase in the presence of an acidic catalyst.
In this reaction, the excess use of a lower olefin is advantageous in view of the conversion of the lower aliphatic carboxylic acid and, therefore, the lower olefin is usually used in excess. Accordingly, it is essential in industry to recycle the lower olefin used in excess.
The pressure during the reaction is preferably higher because this is advantageous in view of the addition reaction and, therefore, a pressure as high as possible is suitably used. In this case, it is easily understood that a process performed while maintaining the entire reaction system including the above-described recycling system of the lower olefin at a high pressure (hereinafter simply referred to a xe2x80x9creaction process under pressurexe2x80x9d) is preferred.
As already described above, the lower aliphatic carboxylic acid introduced into the reaction system must be gasified so as to avoid a risk that the effective component flows out from the catalyst. In the reaction process under pressure, the gasified lower aliphatic carboxylic acid which is generated must have a pressure at least equal to, or higher than, the pressure within the system.
However, the lower aliphatic carboxylic acid generally has a large heat of vaporization and a large amount of energy is required to completely gasify it. Under high pressure, the heat of vaporization necessary for gasifying the lower aliphatic carboxylic acid is even larger. Thus, the reaction process under pressure causes a problem that a very large amount of energy is required.
On the other hand, the lower aliphatic carboxylic acid cannot attain a conversion of 100% in the above-described reaction even if a large excess of the lower olefin is used. Therefore, the process is usually designed to also recycle the unreacted lower aliphatic carboxylic acid.
However, the unreacted lower aliphatic carboxylic acid contains undesired by-products, in many cases. Specific examples of the undesired by-product include hydrocarbon compounds derived from the lower olefin, and carboxylic acid esters resulting from the reaction between the hydrocarbon compounds and the lower aliphatic carboxylic acid.
These by-products are generally unstable at high temperatures and, when the vaporizer is set to a high temperature to obtain a high-pressure gaseous low aliphatic carboxylic acid necessary for the reaction process under pressure, the by-products are decomposed by the heat and may cause a fouling of the vaporizer. Furthermore, when the starting material lower carboxylic acid has corrosiveness, such as formic acid and acetic acid, the corrosiveness is intensified under high pressure and high temperature and, thus, corrosion of the vaporizer arises as a problem.
Further, in the case where the lower aliphatic carboxylic acid has polymerizability, such as acrylic acid or methacrylic acid, exposure, per se, of the acid to a high temperature is undesirable.
As described above, the production process of a lower aliphatic ester, where a corresponding lower aliphatic ester is produced from a lower olefin and a lower carboxylic acid in a gas phase in the presence of an acidic catalyst, is expected to encounter problems such as deactivation of the catalyst resulting from the catalytic component flowing out from the supporter when the reaction mixture in a liquid droplet form, namely, in a mist state, contacts the solid catalyst, or reduction in the reaction results due to unpredictable side reactions caused by the inclusion of the catalytic components in the product.
In order to avoid such a risk and stably perform the reaction over a long period of time, the raw material compound which is liquid at an ordinary temperature must be completely gasified at the reaction using the catalyst. However, to elevate the temperature of the vaporizer of the low aliphatic carboxylic acid for attaining the complete gasification is not preferred not only from the point of view of energy cost but also in view of the side reaction due to the presence of impurities in the recycled lower aliphatic carboxylic acid or of the stability of the highly reactive lower aliphatic carboxylic acid itself.
These problems have, however, not been studied in conventional techniques.
It is an object of the present invention to provide a process for efficiently obtaining a mixed gas of a lower olefin and a lower aliphatic carboxylic acid by gasifying a lower aliphatic carboxylic acid, which is generally difficult to gasify because of its high boiling point and large heat of vaporization, at a relatively low temperature.
It is another object of the present invention to provide a process for producing a lower aliphatic ester, wherein a corresponding lower aliphatic ester is produced from a lower olefin and a lower carboxylic acid in a gas phase in the presence of an acidic catalyst such as a heteropolyacid, which has a particularly excellent effect on energy cost.
In order to attain the above-described objects, the present inventors have made extensive studies particularly on the method for efficiently gasifying a lower aliphatic carboxylic acid and, in turn, on the method for efficiently obtaining a mixed gas of a lower olefin and a lower aliphatic carboxylic acid, in the production process of a lower aliphatic ester where a lower aliphatic ester is produced from a lower olefin and a lower carboxylic acid in a gas phase in the presence of an acidic catalyst.
As a result, it has been found that when a lower aliphatic carboxylic acid is mixed with a lower olefin under an increased pressure and then gasified after heating, the temperature necessary for the gasification of the lower aliphatic carboxylic acid can be greatly reduced and thereby a mixed gas of a lower olefin and a lower aliphatic carboxylic acid can be efficiently obtained. The present invention has been accomplished based on this finding.
More specifically, the present invention provides a process for efficiently producing a mixed gas of a lower olefin and a lower aliphatic carboxylic acid, comprising mixing a lower aliphatic carboxylic acid with a lower olefin under increased pressure and then gasifying the mixture after heating.
The present invention also provides a process, for producing a lower aliphatic ester, comprising reacting a mixed gas of a lower olefin and a lower aliphatic carboxylic acid, obtained by the above-mentioned process of the present invention, in a gas phase in the presence of an acidic catalyst.